Rechargeable and dischargeable secondary batteries have been used for hybrid cars, electric vehicles, electric assist bicycles and others.
A secondary battery is formed by inserting and sealing an electric cell assembly and electrolyte in a container, and the electric cell assembly has positive electrodes and negative electrodes which are stacked on each other with separators of polyethylene, polypropylene, or the like interposed therebetween.
In the case of the secondary battery constructed as above, if, for example, a sharp metal object such as a nail etc., penetrates the container, the positive and negative electrodes may be short-circuited so that an excessive amount of current flows and the battery generates heat which leads to the risk that the battery may explode or catch fire.
To solve this problem, as one example of the related art, Patent Document 1 discloses a method of preventing short-circuit between the positive electrode and the negative electrode if a metal object penetrates the battery container. Specifically, a high-polymer sheet having an coefficient of elongation 1% or greater is disposed on the external surface of the container, so that the high-polymer sheet can stretch along the sides of a metal object that has penetrated the interior of the container so as to cover the metal object. Therefore, the high-polymer sheet that has an extremely low electric conductivity is interposed between the positive and negative electrodes which makes it possible to prevent a short-circuit from occurring between the positive and negative electrodes, thus preventing the battery from exploding or catching fire.